The phosphodiesterase inhibitor, 3-Isobutyl-l-methylxanthine, (MIX) was applied while recording intracellularly from single cones in a perfused eyecup preparation. This depolarized the cone membrane in the dark, decreased amplitude of responses to dim light, enhanced amplitude and slowed time to bright flashes. Results are being analyzed using the model of Baylor, Hodgkin and Lamb.